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Thursday, 8 March 2012

Sunset watchers at the summit of Maunakea show us their tail lights as they leave and the dying colours fade from the sky, on Night Three (out of four) of our attempts to observe Comet Garradd.

Tonight we have decided to come to the summit early, ahead of our start time of midnight, because there is the possibility of ice forming on the road later and my University College London colleague Bob Barber is not so confident of getting us back down safely if he has to drive. So we are relying on our trusty telescope operator, Lucas Fuhrman, to get us safely down to the Hale Pohaku dormitories if we have to leave in a hurry.

Things are looking up, but are by no means great, as you can see by the clouds that are hovering above the Gemini North telescope dome. The relative humidity is falling and the winds are much less than the previous two nights, so we are hoping that the clouds will disperse by midnight so we can observe.

In view of what will still be tricky observing conditions, Bob and I have been discussing our observing strategies with our Gemini support scientist, the legendary Tom Geballe. Tom has been working on the mountain since 1981, first as a support scientist for the United Kingdom Infrared Telescope (UKIRT) and then as UKIRT's director. (If you want to read more about UKIRT and Tom and his achievements - or a very small selection of them - you might want to check out a book I wrote recently.)

What Tom does not know about observing at Mauna Kea probably is not worth knowing. He has helped more people than he can count, and published huge numbers of scientific papers. Indeed, he was actually kidnapped by a "cowboy" posse on his way to a conference dinner and charged with publishing too much to the "detriment" of his colleagues' careers.

Tom has advised us to observe at the longer of the two wavelength settings we have chosen, as that is less affected by high concentrations of water vapour in the atmosphere. This is one of the joys of what is now called "classical observing" where the astronomers get to go to the summit and interact directly with the telescope operator and scientific advisors. Young astronomers who only get to observe by remote polycom interfaces are missing an important and formative experience, in my view, even if they may have to suffer a little altitude sickness for the pleasure.

Shifting our observations to longer wavelengths means we will miss some key "hot" water spectral lines that we have observed in previous comets. So direct comparisons will be more difficult. But it means that we will see other "hot" water lines together with those of ammonia, hydrogen cyanide and even acetylene.

The ratio of water ice to ammonia ice in Comet Garradd is an interesting measurement, that can help us to understand how and where the comet formed, and what the Solar System was like 4.5 billion years ago. The early Sun kept the Solar System much hotter than it is now, and all of the "volatile" compounds, like water and ammonia, would have been boiled off from the proto-Earth. Not until the orbit of Jupiter was reached - five times as far from the Sun as we are - would the temperature have dropped below freezing, so that appreciable amounts of water ice have been able to form.

A lot of that would have been used up forming the core of Jupiter itself - about 20 Earth masses in total. The rest would have formed into comets and planetessimals, that would have been thrown to the outer reaches of the Solar System by the gravitational impact of Jupiter, which would have been grabbing gas from the Solar Nebula to grow to the giant ~200 Earth masses it is now. But at the orbit of Jupiter it probably would still have been too hot for ammonia ice to form, because ammonia freezes at a lower temperature than water. So a lot of the comets that formed near Jupiter would have had little ammonia ice in them. Comets rich in ammonia probably formed further out, towards or beyond the orbit of Saturn.

Depending on how much ammonia to water we find in Comet Garradd, we will have a handle on two possible scenarios. Either, we think we know when the comet formed, and the ammonia to water ratio will tell us roughly where it formed. Or we think we know where the comet formed, and the ammonia to water ratio will tell us roughly when it formed, because we know the rate at which the Sun, and hence the Solar System, cooled down. Unfortunately, there will be some ambiguity and a certain amount of "you pays yer money and takes yer choice", depending on other evidence that can be brought to bear on the problem.

But that is the joy of planetary science. It is complex, not reductive; it is synthetic as well as analytical. And so it requires minds (much better than mine) that can hold many conflicting pieces of evidence in the balance and then weigh them to makes decisions about what happened when the Solar System was forming.